COMPOSITIONALLY MODULATED ALLOY SYNTHESIS FMP c iby 5

BY ELECTROCHEMICAL DEPOSITION

M R Kalantary, G D Wilcox and D R Gabe

IPTME, Loughborough University of Technolom, LEI 1 3TU, UK " 9 7 z777$ __

Abstract P.9 F" Compositionally modulated alloy (CMA) coatings can be synthesised electrochemically The technique is performed by electrodeposition from a plating bath usually containing two different metal ions The alloy modulation is attained through pulsing the voltage or current between two or more pre-optimised levels A controlled electrolyte agitation can also be used to vary the deposit composition

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On the basis of protecting steel substrate against corrosion, CMA coatings have been produced fi-om a zinc-nickel acid sulphate bath. The CMA coatings produced in this study have been assessed with respect to surface morphology and corrosion resistance. The results will be reported in terms of the significance of the zinc-nickel CMA when compared to a single layered alloy coating. The relative merits of the CMA process will be discussed.

Introduction

Demand for new materials with specific properties is ever increasing. One such group of materials is compositionally modulated alloy (CMA) coatings. CMA coatings can be formed by electrochemical deposition on a regular and periodic pattern that forms a laminated coating of two (or more) different materials [ 1-31, Depending upon the application, two or more layers can be formed to give a superior alloy having enhanced properties ranging from mechanical, electrical and magnetic properties to corrosion resistance. Some typical multilayered applications and alloy systems are as follows:

Typical Applications Alloy Systems Protect iveDecorative Electrical & Electronics Strength, Yield & Wear BuildindStructure Protective

Nickel or Copper alloys Cu-Ni, Pd-Ni, Sn-Pb, Au alloys Ni, Co, W, Mo alloys Zinc alloy (Zn-Ni, Zn-Mn, Zn-Co, Zn-Fe)

CMA coatings can be produced electrochemically by two methods: a) Dual Bath technique This involves a combination of metals that can be cathodically reduced to the metallic form, and hence deposited on a metal substrate. Electrodeposition is achieved by subjecting a cathode to two electroplating solutions. Multilayered coatings are produced by switching from one plating bath to the other. Agitation, by means of air bubbling or electrode movement is used to enhance the deposit properties in terms of degree of homogeneity, uniformity and also to preventing pitting. b) Single bath technique This is preferred commercially and consists of deposition from an electrolyte containing two different metal ions in solution. Their deposition potentials such as cathodic overpotential and reverisble potential needed to be far enough apart

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to allow the near separate deposition of each metal. In this technique, a pulse plating process is used to control either potential or current. At higher overpotential or current, the less noble metal, and at lower overpotential or current the more noble metal, are deposited. A controlled electrolyte agitation is also used to vary the deposit composition [ 1-71, The latter associated with mass transfer phenomenon.

The major advantages of the dual bath technique is its simplicity in terms of solution chemistry and its advantage in being able to produce alternate layers of pure metals, as reported for the copper-nickel system by many investigators. The disadvantages of the dual bath technique are i) the difficulty of its use for mass production since it is more labour intensive, ii) more space is required and iii) cross bath contamination is also possible As well as this, some systems such as zinc-nickel will be difficult to plate in alternate fashion since the zinc will corrode preferentially during electrodeposition of nickel. Such corrosion will result in contamination of the nickel plating bath. It must be noted that for the industrial plating of nickel on zinc (e.g. zinc die castings), an electroless copper followed by an electrolytic copper is a common practice prior to nickel plating. Therefore a multilayered coating alternating between pure zinc and pure nickel may be difficult, indicating that perhaps for some systems the dual bath technique has its limitations. The single bath technique has its disadvantages too, where pure layer deposits are often almost impossible to produce. However, it is possible to yield layers of rich deposit,

The difficulties in production of a high purity deposit for each layer fiom a single electrolyte are even more difficult for zinc alloy systems such as zinc-nickel. In this system because of the mechanism of the electrochemical/surface reactions associated with anomalous co-deposition, the deposition of the more noble metal (nickel) is retarded due to the formation of zinc hydroxide and hence preferential deposition of the zinc takes place. Therefore, in the production of zinc-nickel CMA coatings the composition at each layer is an alloy in itself whereas in copper-nickel CMA coatings, layers are much richer (approximately 90-99% nickel or copper). Hence, in the production of CMA coatings from a zinc-nickel sulphate based single bath system the deposits would embody a stack of layers that may contain nickel at lower concentration (up to approximately 30%) than zinc, but sufficient to give both noble and base deposit characteristics with respect to the substrate metal (iron). Such design of zinc-nickel CMA coatings can offer a corrosion resistance property superior to conventional single layer coatings.

The majority of CMA type coatings have been carried out on alloy systems such as copper-lead [ 11, copper-nickel [2,3], copper-cobalt [4], iron-nickel [5] and silver- palladium [ 6 ] . However, the authors are not aware of previous CMA work on zinc- alloy systems, where there may be significant commercial opportunity.

Previous work [7] on the fbndamental aspects of CMA coatings fiom a zinc-nickel alloy electrolyte, has shown that by controlling the mass transport and applied cathodic current the composition of the deposit can vary significantly. Under the different experimental conditions (0.5-1 30 A/dm2 and 0-1 500rpm) studied, it was found that at high current density (1 30A/dm2) and low mass transport the nickel composition in the deposit reached 30%, whereas at the same current density and at higher mass transport the nickel composition dropped by almost a third. It was also revealed that by

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lowering and increasing the current the nickel concentration of the deposit can also vary from 6 to 30%. Therefore, from these established operating conditions for current density and electrode agitation for single layer deposits it was possible to progress to multilayer CMA coatings. In this paper, modulation of the cathodic current using a pulse technique has been used to produce CMA coatings and the appearance, surface morphology and corrosion properties of the CMA coatings for varying individual layer thicknesses have been assessed.

Experimental

Iron cylinders (99.5%) with an outer diameter of 25 mm were used as the substrate material. The samples were abraded with silicon carbide paper to 600 grit. This gave a smooth surface finish, virtually free of rolling marks. Degreasing was carried out in an alkaline cleaner and then samples were etched in 50% v/v S.G. 1 . I8 hydrochloric acid for one minute at 23OC. Subsequently, after cleaning, electrodeposition trials were performed galvanostatically on the cylinder electrodes at 23°C in an electrolyte having, 35 g I-' zinc as zinc sulphate, 35 g I-' nickel as nickel sulphate and 80 g I-' sodium sulphate. The electrolyte pH was 2 (this was adjusted using dilute sulphuric acid). A cylindrical 3 16L stainless steel foil was used as the counter electrode (anode). Compositionally modulated alloy electrodeposits having different layer thicknesses were produced by a pulse current techniques. The total thickness of all samples was eight microns. The sample identification and electrodeposition conditions are shown in Table 1

As a basis for comparison of corrosion performance, two sets of samples have electrodeposoited at 5 and 10 Ndm2 in an electrolyte containing 35 g/I zinc as zinc sulphate, 80 gA sodium sulphate at pH 2.5. A cylindrical zinc foil was used as the counter electrode (anode). Substrate (cathode) agitation was carried out at 100 rev. per minute. The corrosion resistance of these samples was then compared to the single layer and multilayer (CMA) zinc-nickel coated samples mentioned earlier.

The compositionally modulated alloy coatings produced were also characterised by their top layer surface appearance, cross sectional and top surface composition and surface morphology

The surface morphology and compositional analysis of the CMA and single layer samples were carried out using a scanning electron microscope (SEM) and an associated energy dispersive x-ray microanalyser respectively. The corrosion tests were carried out in a 5% wt/wt neutral salt spray according to ASTM B 1 17.

Results

Appearance The surface appearance of the CMA coatings has been found to be generally similar to that of a single layer coating At low current density ( 10A/dm2) the deposit was found to be bright and at high current density (50A/dm2) the deposit appearance was found to be a dark grey colour.

Surface morphology 225

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The surface morphology of the coating produced at I O and 50 A/dm2 shown in Fig. 1 The surface morphology of the top layer deposit, plated at 10A/dm2 generally exhibited needle-like crystal structure whereas surface plated at 50 Ndm2 generally showed more of a clustered nodular morphology. In general, the thinner the individual layers are the greater the domination of the needle-like crystal structure.

The scanning electron micrograph of the cross-section of a compositionally modulated alloy shown in Fig. 2 confirms that the deposit consists of a layered structure.

Compositional analysis To confirm the deposit composition of the CMA coatings samples were prepared by conventional metallography and the cross-section of the samples (at each layer) was analysed by EDX. The results obtained were compared with the analysis of the top surface of the samples and it was apparent that they generally agree with the surface composition of the appropriate single layer deposit as shown in Table 2.

Salt spray corrosion test The results of the salt spray corrosion test for the CMA coated samples and single layer zinc alloy deposits are shown in Table 3 . After 240 hours of salt spray test the CMA coatings having a first layer deposited at 10 Ndm2 or 50 A/dm2 produced better corrosion results (except the duplex deposit having high nickel content adjacent to steel substrate) than the single layer deposits.

Corrosion potential measurements The corrosion potential of the samples was measured in 5% wt/wt sodium chloride solution after exposure to salt spray corrosion testing. The results for CMA coated and single layered zinc-nickel alloy coated samples are shown in Table 3 . The corrosion potentials of the coatings were found to be different. The deposit plated at 10A/dm2 produced a higher corrosion potential (more negative potential) in comparison to that of the deposit plated at 50 A/dm2. Hence the latter deposit is noble and less active than the former.

Discussion

Pulse current electrochemical deposition facilitates the production of the compositionally modulated alloy (CMA) structures. Using this technique by switching from one pre-optimised current level to another it was possible to artificially produce layered alloy coatings having different thicknesses, compositions and structure [ 71. As will be described below this has a significant effect on the deposit properties in terms of corrosion resistance. This result is probably due to differences in the electrochemical potential across the layered coatings.

For cosmetic reason a bright electrodeposit is always more acceptable than a dull one. However, once the coating is covered with a top coat such a passivation treatment and/or paint therefore there would be no draw-back provided properties are superior to a conventionally produced coating. The surface appearance of the CMA coatings has generally been found to be similar to that of a single layer coating. At low current density ( 1 OA/dm2) the deposit was found to be bright and at high current density (50A/dm2) the deposit appearance was found to be a dark grey colour. If a bright

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colour was specified industrially, the advantage of the CMA coating can be extended by manipulation of the top layer coating, giving it a very thin layer of a bright deposit by switching to a suitable current density.

The previous work [7] on the simulation of high speed electrodeposition for the production of the zinc-nickel CMA coatings has revealed that there is little or no effect on deposit enhancement property that could be attributed to the surface morphology. From the results obtained here (Fig. 2) it is apparent that the composition is a major factor influencing the deposit properties and the deposit morphology is the characteristic result of the effect of current density on the structural appearance

The morphology of the cross-section of the zinc-nickel CMA coated sample in Fig. 2 confirms that the deposit consists of layered structures. Further confirmation has been given below by the results of compositional analysis of the individual layer.

The composition of the CMA coatings has a major affect on the overall deposit properties such as mechanical or corrosion resistance [7]. For this reason a selected sample prepared by conventional metallography had each layer compositionally analysed. The result of the analysis of the CMA coatings shown in Table 2, confirms that the nickel concentration in each layer is similar to that of a similar single layer deposit. Hence the electrodeposited produced CMA coatings appear to have the ability to be produced with a definable layer composition profile.

The results of the salt spray corrosion tests for the zinc only deposits, single layer zinc- nickel alloys and zinc-nickel CMA coated samples are shown in Table 3 The result of corrosion tests for the iron substrate has also been included for comparison. It is interesting to find out that there is a greater corrosion resistance property with zinc deposited samples prepared at 5 than 10 A/dm2. Better corrosion resistance at 5 Ndm2 can be attributed to a more compact crystal structure The corrosion test results for zinc-nickel alloy coatings were found to be superior to that of zinc alone. the single layer zinc alloy samples deposited at 10 Ndm2 and 50 Ndm2 produced 20% red rust after 240 hours whereas zinc alone produced 50% red rust after 48 hours of salt spray testing. Therefore the zinc-nickel alloy coatings are superior to that of a zinc deposit alone.

The corrosion resistance of the single layer zinc-nickel alloy coatings produced at 10 A/dm2 and 50 Ndm2 shown similar corrosion resistance property. This agrees with the results reported elsewhere [8] that the deposits provide a sacrificial protection on steel substrate when the composition is up to around 14% nickel.

From the results of salt spray corrosion test it is apparent that, in general the CMA coatings produced a better corrosion resistance property in comparison to single layer coatings. However, amongst all the layered coatings produced the corrosion resistance of the duplex deposit, which had the high nickel-content layer adjacent to the substrate, showed an inferior performance over the single layer deposit This indicates that a) for duplex coatings, the first layer adjacent to the substrate should be less noble, b) when the deposit consists of a repeated layers the corrosion potential of the overall deposit has a less adverse effect than it would with a duplex layer which has a similar layer composition. From these results it can be concluded that for a given 227

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thickness the corrosion property of the CMA coatings is superior to their single layer equivalents; this warrants further investigation and development.

The corrosion potential is an indication of the activity of the deposit in a corrosive environment. M e r exposure to the salt spray corrosion test, the corrosion potential of the CMA and single layer coated samples were measured in a 5% wt/wt sodium chloride solution. The corrosion potentials of all the samples are shown in Table 3 . As it was expected the corrosion potentials of the coatings were found to be different. The zinc deposit alone showed higher corrosion potential (more negative potential) in comparison to that of single layer zinc-nickel alloy samples. This suggests that the corrosion potential for the zinc-nickel alloy is a mixed potential. Further confirmation is the results of zinc-nickel alloy plated at 10 and 50 A/dm2, which shown the corrosion potential of 1.093 V vs. SCE and 0.919 V vs. SCE receptively. The greater the concentration of the nickel in the deposit (see Table 2) the lower the corrosion potential and hence the more noble the deposit will be.

The corrosion potential of the CMA coated samples having a top layer deposited at 1 OA/dm2 showed a higher negative corrosion potential in comparisons to those deposits having been electrodeposited with a top layer at 50A/dm2. It is very interesting to note that the thinner the layer the more active or higher negative potential. However, the long-term corrosion potential increases at a slower rate. The salt spray tests results also appeared to show a better result for the CMA coatings with a high number of repeated layers. Hence under the conditions studied the greater the number of layers in a CMA coating, the better the deposits corrosion resistance. Although there is little or no known published work on the corrosion property of these types of CMA coatings, the conclusions here are very promising. For fbture work CMA coatings containing a greater number of layers will be produced and the corrosion property of the coatings will be assessed to see they produce enhanced properties

Conclusions

1 that of a single layer coating

The surface appearance of the CMA coatings has been found to be generally similar to

2 the composition of the deposit at each layer of the CMA coated samples approximately correspond to that of discrete single layer coatings.

From the Energy Dispersive X-ray Microanalysis (EDX), it has been confirmed that

3 The surface morphology of the top layer deposit, plated at 1 OA/dm2 was generally dominated by a needle-like crystal structure whereas those plated at 50 A/dm2 generally show more of a clustered nodular morphology. The thinner the coatings, the greater the domination of the needle-like crystal structure.

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The CMA coatings, in general, produced better corrosion resistance than single layer

5 the corrosion resistance of the total coating system.

Under the conditions studied, the higher the number of repeated multilayers, the better

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Acknowledgement This work has been fbnded as SERC Project No. GR/H 98007 in the UK

References 1 2

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A R Despic and V D Jovic, J Electrochem SOC 134, 3004-301 I , 1987 A Haseeb, J P Celis and J R Roos, Transaction of Metal Finishers Association of India, 1(3), 15-28, 1992 A R Despic and V D Jovic and S Spaic, J Electrochem SOC , 136, 165 1-1657, 1989 D S Lashmore, R Oberle and M P Dariel, AESF Third International Pulse Plating Symposium, Washington DC, October 28-29, 1986 D L Grimmett, M Schwartz and K Nobe, J Electrochem SOC , 140,973-978, 1993 U Cohen, F B Koch and R Sard, J Electrochem SOC , 130, 1987- 1994, 1983 M R Kalantary, G D Wilcox and D R Gabe, Simulation of high speed electroplating from a single electrolyte for the production of compositionally modulated alloy, Electrochimica Acta, To be published, 1995 M R Kalantary, Plat Surf Fin , 81(6), 80-88, 1994 8

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Table 1: Single layer and CMA coated sample identification

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10 R8, 10150 R4, 2 layers each 4 microns in thickness 10/50 R2, 4 layers each 2 microns in thickness 10/50 R1, 8 layers each 1 microns in thickness

1 layer, 8 microns in thickness

50 R8, 50/10 R4, 2 layers each 4 microns in thickness 50/10 R2, 4 layers each 2 microns in thickness 50/10 R1, 8 layers each 1 microns in thickness

1 layer, 8 microns in thickness

Note: 10= 10 A/&’ SO= SO Ndin‘ R4= each layer 4 microns in thickness lO/SO= Alternate layers were deposited starting at 10 Ndiii2 50/ 10= Alternate layers were produced starting at SO A/din’ DC= Electrodeposition by direct current. The remainder of the saiiiples were electrodeposited by pulse current technique Electrode speed=lOO rpm

Table 2: Concentration of nickel at the top surfaces of the samples and in each layer of selected CMA coated samples

Normalised Values of Nickel composition (Wt %)

Sample Identification Cross sectional analysis S 11 rface Composition of nickel at each layer, startlng from analysis the first layer adjacent to the substrate

1 2 3 4 5 6 7 8 Layer Number

10 R8 8 7 10/50 R4 8 20 16 10/50 R2 15 17 20 10 18 10/50 R1 13 24 10 26 10 20 10 20 14 50 R8 15 50/10 R4 8 50/10 R2 8 50/10 R1 9

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Table 3: Estimated corrosion test results of pure zinc deposit, simple alloy system and compositionally modulated alloys after salt spray test and

subsequent corrosion potential measurements in S% wt/wt sodium chloride solution

Sample Estimated Corrosion Rate, Corrosion Potential (Ecorr= -V) in Number during Salt Spray Test 5% wt/wt Sodium Chloride Solution

ASTM B 117 (After salt spray test)

4lhrs 48hrs 104hrs 240hrs 3hrs 24hrs 4lhrs 48hrs

Fe RR flaking off after 24 hrs 0.653 0.627 0.598

1.116 1.028 0.596 1.118 1.493 0.953 0.649 _ _ _ - ~

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1 OR8 wc 1OYoRR 20YoRR 1.093 0.934 0.726 0.634

I050R4 we ~YoRR 5-10%RR 0.643 0.871 0.621 0.623 1050R2 wc 5%RR 5-1O%RR 0.653 0.915 0.639 0.616 1050R1 wc <5%RR <5yoRR 0.663 0.933 0.647 0.615

50R8 wc lO%RR 20YoRR 0.919 0637 0621 0589

50 1 OR4 WC lO%RR 25%RR 1 016 0 685 0 622 0 596

5010R1 wc G%RR <5%m 1028 0 729 0 643 0629 50 1 OR2 we 5-10°/oRR 10ObRR 1 015 0 709 0 650 0 620

RR- Red Rust ZincSDC- Zinc deposit alone, current density 5 N d m

WC- White Corrosion 2

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Fig. 1 Typical morphology of cross-section of CMA coating showing four distinct layers (1050R2).

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1 OR8

O S 0 R4

OS0 R2

150 R1

Fig. 2 The surface morphology of the top surface of the CMA coatings shows that the deposits top surface has similar characteristics to a single layer coatings.

5010 R4

SO10 R2

5010 R1

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